This invention relates to an apparatus for preparing a blood smeared microscope slide for analysis and more particularly to spinning the microscope slide so as to produce the highest quality blood smear.
In the analysis of blood samples, the blood is smeared on a laboratory slide and the smear is stained. By counting the leukocytes on the stained smear, laboratory technicians performed what is referred to as a white blood cell differential. Automation of this differential has significant economic impact because the differential is performed very frequently at every hospital. A thesis by J. W. Bacus, "An Automated Classification of the Peripheral Blood Leukocytes by Means of Digital Image Processing", University of Illinois, Chicago, 1971, describes one automated system.
U.S. Pat. No. 3,883,852 issued to Douglas A. Cotter, entitled "Image Scanning Converter for Automated Slide Analysis" describes a system developed by my co-workers for automatically scanning and determining the relative number of different types of leukocytes on a stained smear.
Centrifugally spinning a blood wetted slide to produce a monolayer blood film is described in a paper by M. Ingram and F. M. Minter, entitled "Semi-Automatic Preparation of Cover Glass Blood Smears Using a Centrifugal Device", Amer. J. Clin. Path. 51: 214-221, 1969. The method described in this paper includes flooding a cover glass with a layer of blood and centrifuging the cover glass rapidly in a plane parallel to the plane of rotation of the centrifuge. Excess blood is spun off leaving a monolayer of well spread blood cells on the cover glass. Centrifuges for spinning blood smear slides are commercially available. Such devices are available from: Plat General Corporation, (sold by PEI, Incorporated, Abington, Pa.); Perkin-Elmer Corporation, Wilton, Conn.; and Shandon Scientific Company Incorporated, Sewickley, Pa.
In using some of the commercially available centrifuges and blood spinning techniques described above, it has been found that the separation of the red cells was not the same for all blood samples. For some bloods the spinning resulted in blood films with sparsely populated areas interspersed with clumps of cells. For other bloods the technique produced a slide with overlapping cells. As mentioned in the article by Ingram, the morphology of the red cells was often altered. The cells appeared overly flattened and noncircular. Often, white blood cells, specifically neutrophils, appeared damaged. For the blood film to be uniform, a large quantity of blood had to be used. Typically, the surface was flooded prior to spinning. If the entire surface was not wetted an irregular "sunburst" pattern of the blood resulted. Further, it has been found that prior art centrifuges and methods generated artifactual target cells. Target cells which are not true target cells are believed to be caused by air rushing over the slide in such a way as to roll and tumble the red cells in the blood film while the slide is spinning. This rolling and tumbling stretches the membrane of the red cell, resulting in target cells. Manual methods for obtaining a blood smear, wedge and cover-slide method, require a skilled operator, are not very reproducible, and produce distributions which are nonuniform, often containing a high percentage of damaged cells.
U.S. Pat. Nos. 3,577,267 issued to Preston et al. and 3,705,048 issued to Staunton describe centrifuges which can be used to prepare blood slides but the apparatus described in these patents does not solve the problem of producing blood smears with good cell morphology, good cell distribution, and absence of artifactual target cells for all blood samples.